A conventional multistage front chain gear comprising a gear body molded of synthetic resin is both more lightweight and also larger in buffer action with respect to a driving chain than gears formed of metal, such as iron or aluminum, during the speed change and drive of the bicycle. Thus, such gears having gear bodies molded of synthetic resin are advantageous in that noises generated between the gear and the chain during the bicycle's running can be reduced and the toothed body can have increased ductility when subjected to a driving force, in turn imparting less shock to a cyclist.
The chain, when stretched between the multistage front chain gear and the multistage rear chain gears, especially between the larger diameter toothed body (top gear) at the front chain gear and the larger diameter rear chain gear (low gear) and between the smaller diameter toothed body (low gear) at the front chain gear and the smaller diameter rear chain body (top gear), moves obliquely to a maximum with respect to the center line of the width dimension of the bicycle so that the inner surface of a link plate at the chain comes into contact with the outside of the larger diameter toothed body with respect to the center line of the width dimension of the bicycle and with the inside of the smaller diameter toothed body with respect to said center line in an inclining manner. Also, a front gear formed of synthetic resin is smaller in cutting resistance than one formed of iron or aluminum, so that the outside of the larger diameter toothed body and the inside of the smaller diameter body are susceptable to being cut by the chain, resulting in a short life span of the gear.
The countermeasure for the above has been to apply a metal-plating onto the surface of the chain gear of synthetic resin for increasing the cutting resistance. Such metal plating, however, requires surface treatment for degreasing, so that the manufacturing process is complicated as a whole and the manufacturing cost is high. Furthermore, when the plated metal film is made thicker in order to increase the cutting resistance, the teeth at the toothed body may be deformed and also a film may be breakable so that the gear of sufficient cutting resistance cannot be obtained. Also, the synthetic resin selected for use is limited to resins suitable for metal plating, with the result that a desired synthetic resin superior in buffer action and ductility cannot be selected.